Fix regression with webpack

Add some entries to `package.json` to improve compatibility with webpack

- Add `import` export as per webpack requirements
- Add `style` export for CSS file
- Add `*.css` to "side effects" to prevent issues with tree-shaking

With this commit, it is possible to import frappe-gantt without breaking
changes in webpack-based applications

Refs:
- https://webpack.js.org/guides/package-exports/

Close #439

package.json

@@ -26,7 +26,9 @@
     ],
     "exports": {
         ".": {
-            "require": "./dist/frappe-gantt.umd.js"
+            "require": "./dist/frappe-gantt.umd.js",
+            "import": "./dist/frappe-gantt.es.js",
+            "style": "./dist/frappe-gantt.css"
         }
     },
     "keywords": [
@@ -55,5 +57,8 @@
     "eslintIgnore": [
         "dist"
     ],
+    "sideEffects": [
+      "*.css"
+    ],
     "packageManager": "[email protected]"
 }